1. Field of the Invention
The present invention relates to a CDMA/TDD radio communication system.
2. Description of the Related Art
Demand for land mobile communication devices such as car telephones and portable telephones is increasing remarkably, and a frequency effective utilization technique for securing larger subscriber capacity in a limited frequency band have become important in recent years. A code-division multiple access (CDMA) system attracts attention as one of multiple access systems for frequency effective utilization. In the CDMA system, when a signal level from a mobile unit near a base station is high in a reverse link from the mobile unit to the base station, such a near-far problem that a signal from another mobile unit become unreceivable is generated. Thus, it is required to control transmission power of the mobile unit so that the base station receives the signal from any mobile unit at the same level.
On the other hand, since propagation conditions both for transmission and reception are the same in a time division duplex (TDD) system in which transmission and reception are performed in the same frequency band, when the propagation condition on one side is known, the propagation condition on the other side can be known. With this, it is possible to measure the reception power at the mobile unit and to control the transmission power of the mobile unit based on the measured value.
Transmission power control techniques in a conventional CDMA/TDD radio communication system will be described hereinafter with reference to FIG. 1. In FIG. 1, reference numeral 1 represents a base station, 2 a mobile unit, 3 a data channel spreading circuit for spreading a first transmitted data 11 from the base station 1, 4 a despreading circuit for obtaining a first received data 12, 5 a transmission line, 6 a data channel despreading circuit for obtaining a second received data 13 in the mobile unit 2, 7 a data signal reception level measuring circuit for measuring the level of the second received data 13, 8 a transmission power control circuit for controlling the transmission power of the mobile unit in accordance with the reception level, 9 a spreading circuit for spreading a second transmitted data 14 from the mobile unit 2, and 10 a power amplification circuit.
Next, the operation of the conventional example described above will be explained. In the base station 1, the first transmitted data 11 are spread by means of the data channel spreading circuit 3. The spread signal is transmitted to the mobile unit 2 through the transmission line 5 after being added to the spread signals of other channels. The signal received by the mobile unit 2 is despread in the data channel despreading circuit 6 to obtain the second received data 13. In the data signal reception level measuring circuit 7, the reception level is measured with the second received data 13. In the transmission power control circuit 8, attenuation in the transmission line 5 is estimated based on the measured value of the reception level to determine the transmission power when the mobile unit 2 spreads the second transmitted data 14 by the spreading circuit 9 and transmits those data after amplification in the power amplification circuit 10. The data transmitted from the mobile unit 2 through the transmission line 5 are despread by the despreading circuit 4 in the base station 1 to obtain the first received data 12. When the power level of the first transmitted data 11 transmitted from the base station 1 is constant, it is possible to obtain the attenuation of the transmission line 5 accurately, so that it is possible to control the transmission power of the mobile unit 2 accurately.
In the conventional transmission power control techniques described above, however, it is impossible to obtain the attenuation of the transmission line 5 accurately and to control the transmission power accurately when the transmission power level of the first transmitted data 11 changes. Further, the attenuations of the transmission line 5 estimated in respective mobile units 2 are different due to the fact that the contents of the transmission data transmitted from the base station 1 to respective mobile units 2 are different from one another and separate spreading codes are used, respectively, and the transmission power of respective mobile units 2 is determined based on the estimated attenuation of the transmission line 5. Thus, the power arriving at the base station 1 from respective mobile units 2 becomes uneven.